The present invention broadly concerns devices for use in spinal implant systems, particularly those using spinal rods contoured for connection at various locations along the length of the spinal column. More specifically, the invention concerns an apparatus for spanning between spinal rods to support vertebral fixation elements of the implant system which provide direct engagement to vertebrae of the spinal column. The invention is particularly useful with methods and devices for anterior fixation of the spine.
Spinal fractures often occur at the thoracolumbar junction. Most of these fractures are burst injuries which are particularly dangerous because retropulsed bone fragments can cause spinal cord or caudal equina injuries. Posterior fixation has long been the primary approach for traumatic spinal injuries of this type.
The development of posterior internal fixation procedures for burst fractures was a substantial improvement over early approaches of bed rest and body casts. However, several disadvantages to these procedures were discovered. For example, this approach fails to reduce kyphosis or allow complete clearing of the spinal canal. Other complications include pseudoarthroses, late rod disengagement and inadequate reduction. Also, some posterior instrumentations require the fusion to extend at least two levels above and below the injury, particularly at the thoracolumbar junction. The posterior approach is also limited in the viability for use in burst fractures because in such fractures neural compression generally occurs from the anterior direction. Therefore, it is better to decompress and fuse the spine anteriorly. These difficulties have motivated attempts at anterior approaches. Various anterior and posterior spinal fixation devices and methods are discussed in Howard S. An, et al., (1992) Spinal Instrumentation, herein incorporated by reference.
There are several advantages to anterior internal fixation. An anterior approach allows complete clearance from the spinal canal of bone fragments and/or total resection of a tumor. It also permits fusion of a minimal number of motion segments. In spite of these advantages, the use of anterior approaches has been limited by the risk of complications and other disadvantages of current systems.
Several plate and screw systems have been designed for anterior instrumentation of the spinal column. The Syracuse I-Plate (Danek and Synthes) may use rigid or semi-rigid screws in combination with a plate. Distraction or compression of the bone graft is not possible with this system. The Casp Plate marketed by Acromed is designed to be used in a semi-rigid manner. This device, as well, does not permit compression or distraction of the bone graft and in addition cannot be used in a rigid construct. The Stafix Plating System marketed by Daruma of Taipei, Taiwan, is an anterior thoracolumbar plate designed to address similar indications. This plate incorporates slots and holes as well as permitting quadrilateral placement of screws. The Anterior Thoracolumbar Plating System under development with Danek and Dr. Zdeblick is a slotted plate designed to attach to the anterior lateral aspect of the vertebral body. The plate allows distraction and/or compression through the use of two screws and two bolts.
Several modular spinal instrumentation systems were developed for anterior instrumentation. The Kaneda device is a system which includes a rod coupler distant from the point of attachment to the vertebral bodies. Rods are inserted through holes in the spinal screw heads which are then attached to the superior and inferior vertebral bodies. Normally two screws are placed in each body, therefore two rods are required. These rods are threaded to allow compression and distraction and are crosslinked to form a solid construct at the end of the procedure. The Texas Scottish Rite Hospital System is also a modular spinal system which can be used anterioraly for the management for burst fractures or tumors. This device can be configured much in the same way as the Kaneda device with two screws in the superior and inferior vertebral body, each connected by rods which are crosslinked together. The Dunn device is another anterior spinal fixation device for use in tumor or thoracolumbar burst fractures. This device, similar to Kaneda, involves vertebral body staples, screws positioned in the vertebral body, and two threaded rods connecting a superior and inferior vertebral body to form a rigid construct.
These systems have proved unsatisfactory. Many of these devices such as the Syracuse I-plate and the Casp plate do not allow distraction or compression of a bone graft in fusion cases. Such static systems cannot be used to correct certain disorders such as kyphosis. The systems that do allow distraction and/or compression are often too complicated and involve the use of multiple screws and bolts. The prominent bone screws and rods of some devices increase the danger of vascular injury. Hardware failures, such a screw pull-out, have led to complications, including pseudoarthrosis. Some systems are further limited because they cannot be used in a rigid construct.
It would therefore be desirable to have a low profile, streamlined system with a minimum of separately implanted components to reduce the amount of time required to implant the system, the risk of vascular injury and the problem of irritation to the surrounding soft tissue of the patient.
A need exists for devices for anterior fixation which reduce the risks of anterior fixation by providing a mechanism to prevent hardware failures, such as screw pull-out.
It is desirable to have a spinal fixation system that is readily adapted to provide lateral coupling between spinal rods and multiple stages or segments of the spinal column. Such a system should provide this segmental interconnection without interfering with vertebral areas available for bone grafting to achieve permanent fixation or immobilization of damaged vertebrae.
There is also a need for low profile, streamlined systems which allow variation of the distances between vertebrae, i.e., compression and distraction, without the need for complicated instrumentation and tools.
There is currently no system that addresses each of these features in a single apparatus. The present invention addresses these needs and provides other benefits not previously found in spinal fixation systems of the prior art.
In accordance with the invention, an apparatus is provided for spanning between a pair of longitudinal members situated adjacent a patient""s vertebrae along the sagittal plane. The assembly includes a number of connectors which are engageable to the longitudinal members via clamping surfaces provided in a slot defined in the connector. Each of the connectors defines a thru-hole for engaging a bone bolt which in turn is engaged to a vertebral body. A fastener clamps the bone bolt to the connector. The assembly also includes a number of fixation spikes projecting from the connector which are configured to engage the vertebrae.
In a specific embodiment of the invention, there is provided a locking mechanism configured to prevent the bolt from rotating relative to the connector and the vertebra when the nut is being tightened. The locking mechanism may include radial splines on the lower surface of the connector and also on a mating face on the bone bolt. In another embodiment, the spinal fixation system includes a dynamic, or movable, rod connector and a fixed rod connector which allows variation of the distances between vertebrae for compression or distraction.
One object of the invention is to provide an apparatus for use in laterally connecting longitudinal members implanted adjacent a patient""s vertebral column.
Another object of this invention is to provide an apparatus which provides for convenient management of thoracolumbar burst fractures and tumors and which permits anterior load sharing as well as compression and distraction.
One benefit of the apparatus of the present invention is that it combines means for connecting the vertebral fixation elements to the spinal rods with means for laterally or transversely connecting the spinal rods together. An additional benefit is that the invention provides a more compact construct with a lower profile as compared to prior spinal rod constructs employing many individual components to connect vertebrae and spinal rods.
Yet another benefit achieved by the invention resides in providing segmental coupling or connection of the spinal rods, while permitting a wide variation of orientations at the vertebral fixation elements relative to the spinal rods.
Another advantage of this invention is that it provides fixation assemblies that can be top loaded, or implanted over bolts after the bolts have been engaged in the vertebrae.
Other objects and further benefits of the present invention will become apparent to persons of ordinary skill in the art from the following written description and accompanying figures.